John Lindholm, age 12, of Scottsdale,, Ariz., for his question:

Is it true that a diamond can scratch.glass?

The stony minerals are classed according to their hardness. On the scale of mineral hardness, there are ten classes and various shades of difference in each class.    Opal, for example, may be anywhere from 5.5 to 6.5 on the scale of hardness. The softest minerals, such as talc and graphite, are in class    one. Diamond, the hardest of the natural minerals, has class ten all to itself.

A simple test for hardness is scratching. Tale can be scratched by the stones in all the harder classes and it cannot scratch back. A diamond can scratch any stone and no stone can scratch it back. The hardness of window glass, which is mostly quartz, is about 5.5, hence a diamond can scratch it with ease.